Because of changes in ambient temperature, the resistance thermometer circuitry must be compensated. The resistors that are used in the measuring circuitry are selected so that their resistance will remain constant over the range of temperature expected. Temperature compensation is also accomplished through the design of the electronic circuitry to compensate for ambient changes in the equipment cabinet. It is also possible for the resistance of the detector leads to change due to a change in ambient temperature. To compensate for this change, three and four wire RTD circuits are used. In this way, the same amount of lead wire is used in both branches of the bridge circuit, and the change in resistance will be felt on both branches, negating the effects of the change in temperature.

Summary

Temperature detection circuit operation is summarized below.

Circuit Operation Summary

The basic bridge circuit consists of:

Two known resistors (R₁ and R₂)that are used for ratiomg the adjustable and known resistances

One known variable resistor (R₃) that is used to match the unknown variable resistor

One unknown resistor (R_X) that is used to measure temperature

A sensing ammeter that indicates the current flow through the bridge circuit

The bridge circuit is considered balanced when the sensing ammeter reads zero current.

A basic temperature instrument is comprised of:

An RTD for measuring the temperature

A bridge network for converting resistance to voltage

A DC to AC voltage converter to supply an amplifiable AC signal to the amplifier

An AC signal amplifier to amplify the AC signal to a usable level

A balancing motor/mechanical linkage assembly to balance the circuit's resistance

An open circuit in a temperature instrument is indicated by a very high temperature.

A short circuit in a temperature instrument is indicated by a very low temperature.

Temperature instrument ambient temperature compensation is accomplished by:

Measuring circuit resistor selection Electronic circuitry design

Use of three or four wire RTD circuits